Question

What are the issues caused by changes in pressure of gas cap during well startup?

Answer 1

With gas, the Joule Thompson effect may give temperatures far below 0 °C due to high pressure drop over production choke valves during startup. During shutdown of a well, the upper part of the well tubing, will cool down rapidly because the ambient formation temperature is near to the seawater temperature, if the tubing is not thermally insulated and the tubing is gas filled. This can give the following problems: 

• Low temperature material problems 
• Hydrate and/or ice formation 
• Higher than normal hydrate inhibitor injection rate 
• May require hydrate inhibitor injection rate above injection capacity 

During depressurization, severe cooling may be caused by isentropic expansion (lost expansion work for the fluid at the entrance of the pipe which "pushes" i.e. does work on the fluid downstream, due to expansion ) in addition to the Joule-Thompson cooling. This may give temperatures below minimum design. 

Answer 2

Long time to heat up a well. The biggest problem in this respect is to calculate the transient temperature correctly. It can also be a problem to start up because of low temperatures over extended times which requires special precaution with hydrate protection (normally with continuous methanol injection during startup) and with material problems. This problem is increased if there is a need for significant choking during startup.

Long time to heat up a flowline. During startup it may be required with injection of methanol. Alternatively can the line be heated up (e.g. with hot oil circulation or with heat tracing) before well production startup.